Microalgae represent a diverse group of micro-organisms adapted to various ecological habitats (for example, as described in Hu et al., Plant J (2008) vol. 54 (4) pp. 621-639). Many microalgae have the ability to produce substantial amounts (for example, 20-50% dry cell weight) of lipids, such as triacyl glycerols (TAGs) and diacylglycerols (DAGs), as storage lipids under stress conditions, such as nitrogen starvation. Under nitrogen starvation many microalgae exhibit decreased growth rate and break down of photo synthetic components, such as chlorophyll.
Fatty acids, the building blocks for TAGs and all other cellular lipids, are synthesized in the chloroplast using a single set of enzymes, in which acetyl CoA carboxylase (ACCase) is key in regulating fatty acid synthesis rates. However, the expression of genes involved in fatty acid synthesis is poorly understood in microalgae. Synthesis and sequestration of TAGs into cytosolic lipid bodies appears to be a protective mechanism by which algal cells cope with stress conditions.
Little is known about the regulation of lipids, such as TAG formation, at the molecular or cellular level. At the biochemical level, available information about fatty acid and TAG synthetic pathways in algae is still fragmentary. Knowledge regarding both the regulatory and structural genes involved in these pathways and the potential interactions between the pathways is lacking. Because fatty acids are common precursors for the synthesis of both membrane lipids and TAGs, how the algal cell coordinates the distribution of the precursors to the two distinct destinations or the inter-conversion between the two types of lipids needs to be elucidated. Many fundamental biological questions relating to the biosynthesis and regulation of fatty acids and lipids in algae need to be answered.
Much research has been conducted over the last few decades regarding using microalgae as an alternative and renewable source of lipid-rich biomass feedstock for bio fuels. Microalgae are an attractive model in that they are capable of producing substantial amounts of lipids such as TAGs and DAGs under stress conditions, such as nitrogen starvation. However, a decrease in growth of the microalgae under nitrogen starvation makes it harder to use microalgae in the large scale production of biofuels. While algae provide the natural raw material in the form of lipid-rich feedstock, our understanding of the details of lipid metabolism in order to enable the manipulation of the process physiologically and genetically is lacking.
Thus, a need exists to better understand the regulation of lipids, such as TAGs and DAGs, in algae at the molecular level. Furthermore, it would be useful to genetically manipulate algae such, that the algae are capable of producing substantial amounts of lipids without decreased growth rate and the break down of algal components, such as chlorophyll.